Solders are commonly utilized in semiconductor device packaging. If the solders contain alpha particle emitting isotopes (referred to herein as alpha particle emitters), emitted alpha particles can cause damage to packaged semiconductor devices. Accordingly, it is desired to reduce the concentration of alpha particle emitters within the solders.
An exemplary prior art semiconductor package is shown in FIG. 1 as a package 50, with the exemplary package representing a flip-chip construction. The package comprises a semiconductor component 12 (such as, for example, an integrated circuit chip). The package also comprises a board 14 utilized to support the semiconductor component 12. A plurality of contact pads 38 (only some of which are labeled) are joined to chip 12, and a plurality of contact pads 40 (only some of which are labeled) are joined to board 14. Solder balls or bumps 39 (only some of which are labeled) are provided between pads 38 and 40 to form electrical interconnects between pads 38 and 40. The electrical connection utilizing the solder balls or bumps 39 with pads 38 and 40 can incorporate so-called wafer bump technology.
Suitable encapsulant 44 can be provided over the chip 12 and substrate 14 as shown. Additionally, and/or alternatively, thermal transfer devices (not shown) such as heat sinks and heat spreaders can be provided over the chip 12.
Contact pads 30 (only some of which are labeled) are on an underside of the board 14 (i.e., on a side of board 14 in opposing relation relative to the side proximate chip 12). Contact pads 30 typically comprise stacks of copper, nickel and gold. Solder balls 32 (only some of which are labeled) are provided on the contact pads and utilized to form electrical interconnections between the contact pads 30 and other circuitry (not shown) external of the chip package. The contact pads 40 can be connected with pads 30 through circuit traces (not shown) extending through board 14.
The shown package 50 has solder proximate chip 12 from at least balls 39, and possibly through wafer bumps associated with pads 38 and/or pads 40. There can be other applications of solder within package 50 which are not specifically shown. For instance, a solder paste can be provided between chip 12 and various thermal transfer devices.
The solders utilized in package 50 can be problematic, as discussed above, in that the solders can contain alpha particle emitters. Alpha particles are problematic for semiconductor devices because the alpha particles can induce so-called soft errors. The errors are referred to being “soft” in that the errors are not permanent. However, the errors will typically cause at least one round of incorrect calculations.
There are numerous sources for alpha particles, including reactions caused by cosmic rays. However, the source which is frequently most problematic for semiconductor device packages is solder utilized for forming various interconnections relative to semiconductor dies. For instance, the wafer-bump technique is becoming relatively common for forming high density interconnects to semiconductor dies. The bumps are portions of solder formed over electrical nodes associated with a semiconductor die package. If the solder utilized in the bumps has alpha particle emitting components, the alpha particles are frequently emitted close to integrated circuitry associated with the semiconductor die.
Occasionally, the solder formed over the electrical nodes is in the form of large pillars. Such pillars are frequently referred to as columns. For purposes of interpreting this disclosure, the term “bump” is to be understood to encompass various forms of solder formed over electrical nodes, including the forms commonly referred to as columns.
A typical component of many solders is lead. However, one of the lead isotopes (specifically 210Pb) has a decay chain that leads to alpha particles. Further, various common contaminants of lead can emit alpha particles, including, for example, isotopes of uranium, thorium, radium and polonium.
The alpha particle emitters present in lead can be present in the ore from which the lead is initially refined. Alpha particle emitters can be alternatively, or additionally, introduced during processing and/or use of the lead. For instance, phosphoric acid and some antistatic systems contain alpha particle emitters; some abrasives and cleaning agents can introduce alpha particle emitters into lead; and smelting of commercial lead can introduce uranium, thorium and other alpha particle emitters into the lead from gangue rock.
The amount of alpha particle emitters present in lead is typically determined by an alpha flux measurement, with results stated in terms of alpha particle counts per unit area per hour (cts/cm2/hr). It is possible to commercially obtain lead having an alpha flux of from 0.002 to 0.02 cts/cm2/hr, but it is very difficult to obtain a material with a lower alpha flux. However, the semiconductor industry is requesting materials with significantly lower alpha flux, including for example, materials having an alpha flux of less than 0.0001 cts/cm2/hr.
Among the difficulties associated with reducing the concentration of alpha flux emitters in a material to extremely low levels is a difficulty in measuring the concentration of the emitters at flux levels below 0.002 cts/cm2/hr. Unless the concentration can be measured, it is difficult to monitor a purification process to determine if alpha particle emitters are being removed. For instance, it can be difficult to determine at any given stage of the purification process if alpha particle emitters are fractionating with a material or away from the material.
Although the discussion above focuses on removing alpha particle emitters from lead-containing solders, it should be understood that alpha particle emitters are also problematic in other materials. For instance, one of the methods utilized to reduce the concentration of alpha particle emitters in solder has been to create so-called lead-free solders. Such solders contain little, if any, lead, which is desirable from an environmental perspective. However, the solders can still have an undesirable amount of alpha particle emitters present therein. Exemplary lead free solders are Sn: 3.5% Ag; Sn:4% Ag:0.5% Cu; and Bi:2-13% Ag, where the percentages are by weight.
One of the methods which has been utilized for reducing the number of alpha particle emitters in lead-containing solders is to start with lead materials which have very few emitters therein. Presently there are three sources of such materials. The sources are (1) very old lead where the 210Pb has substantially all decayed; (2) some specific PbS ore bodies which have very little 210Pb therein, and which have been carefully refined; and (3) lead which has been subjected to laser isotope separation to remove the 210Pb from the lead. Various problems exist with all of the sources. For instance, the first source utilizes very old Pb, and such is often poorly refined and therefore contains various radionuclides as contaminants. The second source typically does not have a low enough alpha particle emitter concentration to meet the ultimately desired requirements of the semiconductor industry. The third source is very energy intensive to form, and therefore is not commercially feasible.